The present invention relates to an apparatus for generating clock pulses. More specifically, it relates to an apparatus for generating clock pulses of which duty ratio remains stable despite temperature, load capacitance, or supply voltage changes.
Conventional computers use the rising timing of the clock pulses as the reference timing. However, a computer using both the rising and the falling timings of the clock pulses as the reference timing is being developed in order to increase operating speed. The resultant operation speed is twice that of conventional computers, while using clock pulses of conventional frequency. Whereas the rising and falling timing of the clock pulses are used for the reference timing, the clock pulses should have the duty ratio of exactly 50 percent which remains stable and is scarcely disturbed by ambient changes.
In order to obtain clock pulses having a duty ratio of 50 percent, an apparatus as shown in FIG. 9 has been employed. This apparatus is constituted of a crystal oscillator 5 and an amplifier Q2. The crystal oscillator 5 is a Colpitts-type quartz-crystal oscillator including an inverter Q1 made of a Complementary Metal Oxide Semiconductor (C-MOS) integrated circuit and a crystal resonator 1 in parallel with the inverter Q1. A resistor R and a capacitor C are adjustable, in order to effect a duty ratio of 50 percent.
In the apparatus shown in FIG. 9, the circuit must be manually adjusted, otherwise dispersive characteristics of the elements are such that accurate clock pulses having a 50-percent duty ratio will not be emitted.
Another conventional apparatus as shown in FIG. 10 includes an oscillator 2, for generating output having a frequency twice that of the desired clock pulses, and a flip-flop 3 for dividing the output from the oscillator 2 in half. There is propagation delay time in the flip-flop 3 of this apparatus which causes variation of the transition time from low to high level as opposed to the transition time from high to low level in the high frequency range, for example at 50 MHz. Furthermore, the propagation delay time usually changes depending upon the load capacitance of the apparatus. Consequently, clock pulses having an accurate 50-percent duty ratio are difficult to obtain.
Another quartz-crystal oscillator for making steady clock pulses having a 50-percent duty ratio is disclosed in U.S. Pat. No. 4,383,224. However, this apparatus does not sufficiently maintain 50-percent duty ratio, resulting from dispersive characteristics in its elements, and from changes in temperature, load capacitance and supply voltage.